Ceramic sintered product and process for producing the same

ABSTRACT

Low thermal expansion ceramics contains a cordierite crystal phase, wherein a phase of a crystalline compound containing at least one element selected from the group consisting of an alkaline earth element other than Mg, a rare earth element, Ga and In, is precipitated in the grain boundaries of said crystal phase, said ceramics has a relative density of not smaller than 95%, a coefficient of thermal expansion of not larger than 1×10 −6 /° C. at 10 to 40° C., and a Young&#39;s modulus of not smaller than 130 GPa. That is, the ceramics has a small coefficient of thermal expansion, is deformed very little depending upon a change in the temperature, has a very high Young&#39;s modulus and is highly rigid and is resistance against external force such as vibration. Accordingly, the ceramics is very useful as a member for supporting a wafer or an optical system is a lithography apparatus that forms high resolution circuit patterns on a silicon wafer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ceramics that thermally expands littlecontaining cordierite as a main crystal phase. In particular, theinvention relates to ceramics that thermally expands little and isadapted for use in various devices used for a process for producingsemiconductors, such as a semiconductor wafer support fitting like avacuum chuck, succeptor, electrostatic chuck, or a stage or a member forsupporting an optical element in a lithography apparatus.

2. Description of the Prior Art

The cordierite-type sintered product has heretofore been known asceramics that thermally expands little, and has been used for filters,honeycombs and refractories. The cordierite-type sintered product isobtained by using a cordierite powder or a powder in which is mixed MgO,Al₂Do₃ and SiO₂ in amounts capable of forming cordierite, by adding, tothis powder, a sintering assistant such as an oxide of a rare earthelement, SiO₂, CaO or MgO, molding the mixture into a predeterminedshape, and firing the obtained molded article at 1000 to 1400° C.(Japanese Examined patent Publication (Kokoku) No. 3629/1982 andJapanese Unexamined Patent Publication (Kokai) No. 229760/1990).

Various members used for the process for producing semiconductors suchas LSIs, e.g., semiconductor wafer support fittings such as vacuumchuck, succeptor, electrostatic chuck, and a stage and members forsupporting an optical element is a lithography apparatus, haveheretofore been produced by using ceramics such as alumina or siliconnitride on account of the reason that it is chemically stable and isobtained at a reduced cost. Accompanying a trend toward a highintegration degree in the LSIs in recent years, however, high resolutioncircuits have been formed in the semiconductor wafer requiring highdegree of precision. For example, the lines of the circuits have a widthof the order of submicrons. In a lithography apparatus used for formingthe circuits of this kind, the positioning precision required for thestage for holding the wafer in which the circuit is to be formed must be100 nm or smaller. The ceramics such as alumina and silicon nitride haveconsiderably large coefficients of thermal expansion at 10 to 40° C.(5.2×10⁻⁶/° C. in the case of alumina, and 1.5×10⁻⁶/° C. in the case ofsilicon nitride). With such ceramics, a change of 0.1° C. in thetemperature of the atmosphere results in the deformation of aboutseveral hundred nanometers, making it no longer possible to satisfy theabove-mentioned requirement of precision.

It has also been proposed already to apply the cordierite-type sinteredproduct to various parts used for a process for producing semiconductors(Japanese Unexamined Patent Publication (Kokai) No. 191422/1989,Japanese Examined Patent Publication (Kokoku) No. 97675/1994). Thecordierite-type sintered product thermally expands less than theabove-mentioned alumina or silicon nitride, and is favorable form thestandpoint of preventing a drop in the precision of the circuit causedby thermal expansion. This sintered product, however, has low rigiditywhich is a defect. That is, the semiconductor wafer support member suchas a stage in the lithography apparatus moves at a high speed to aregion where the exposure to light is to be executed, stops at apredetermined position and, then, the wafer placed on the support memberis exposed to light. The support member made of the cordierite-typesintered product having a low rigidity develops vibration when it hasstopped moving, and the exposure to light is executed in a vibratingstate, resulting in a drip in the precision of exposure to a conspicuousdegree. The drop in the precision of exposure becomes conspicuous as thelines of the circuit formed by exposure to light become fine, casting afatal problem from the standpoint of forming high resolution circuits.

Moreover, the members supporting the optical elements in the lithographyapparatus transmits vibration to the optical elements accompanying themotion of the stage. When the exposure is effected relying upon suchoptical elements, therefore, the light beam vibrates causing the focalpoint to be blurred or deviated and, eventually, causing the precisionof exposure to be greatly deteriorated.

SUMMARY OF THE INVENTION

The object of the present invention, therefore, is to provide ceramicsthat thermally expands little and has a high rigidity (high Young'smodulus) and a process for producing the same.

Another object of the present invention is to provide cordieriteceramics that thermally expands little, has a high Young's modulus, andcan be effectively used for various members in a process for producingsemiconductors owing to the above-mentioned properties, and a processfor producing the same.

According to the present invention, there is provided low thermalexpansion ceramics containing a cordierite crystal phase, wherein aphase of a crystalline compound containing at least one element selectedfrom the group consisting of an alkaline earth element other than Mg, arare earth element, Ga and In, is precipitated in the grain boundariesof said crystal phase, said ceramics having a relative density of notsmaller than 95%, a coefficient of thermal expansion of not larger than1×10⁻⁶/° C. at 10 to 40° C., and a Young's modulus of not smaller than130 GPa.

According to the present invention, there is further provided a processfor producing low thermal expansion ceramics containing a cordieritecrystal phase, comprising:

-   -   preparing a molded article that contains a cordierite component        and an oxide containing at least one element selected from the        group consisting of an alkaline earth element other than Mg, a        rare earth element, Ga and In, or a compound component capable        of forming said oxide;    -   firing said molded article at a temperature of from 1100° C. to        1500° C. to obtain a sintered product having a relative density        of not smaller than 95%, and    -   cooling said sintered product from at least the firing        temperature down to 1000° C. at a temperature drop rate of not        larger than 10° C./min.

According to the present invention, furthermore, there is provided aprocess for producing ceramics that thermally expand little containing acordierite crystal phase, comprising:

-   -   preparing a molded article that contains a cordierite component        and an oxide containing at least one element selected from the        group consisting of an alkaline earth element other than Mg, a        rate earth element, Ga and In, or a compound component capable        of forming said oxide;    -   firing said molded article at a temperature of form 1300° C. to        1500° C. to obtained a sintered product having a relative        density of not smaller than 905.    -   subjecting said sintered product to a hot hydrostatic treatment        in a pressurized atmosphere of not lower than 100 atms. at a        temperature of form 1100 to 1400° C.; and    -   cooling said sintered product from at least the temperature of        said hot hydrostatic treatment down to 1000° C. at a temperature        drop rate of not larger than 10° C./min.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram schematically illustrating a lithography apparatusused for a process for producing semiconductors.

DETAILED DESCRIPTION OF THE INVENTION

The ceramics of the present invention has a main crystal phase formed ofcordierite and, hence, thermally expands little.

Cordierite is a composite oxide represent ideally by the followingformula,2MgO.2Al₂O₃.5SiO₂  (I)and is present in the form of crystalline particles having an averageparticle diameter of from 1 to 10 μm in the ceramics. The ceramicsthermally expands less as the content of the cordierite crystal phaseincrease. The ceramics of the present invention contains the cordieritecrystal phase in such an amount that the coefficient of thermalexpansion is not larger than 1×10⁻⁶/° C. and, particularly, not largerthan 0.5×10⁻⁶/° C. at 10 to 40° C.

In the present invention, furthermore, it is very important that acrystalline compound containing at least one element selected from thegroup consisting of alkaline earth element other than Mg, rare earthelement, Ga and In, is precipitated on the grain boundaries of thecordierite crystal phase. This prevents a drop in the coefficient ofthermal expansion and, at the same time, helps increase the Young'smodulus.

The above-mentioned element component is used as a sintering assistant,and forms a liquid phase upon reacting with some of the components inthe cordierite during the firing, contributing to enhancing thesintering property. The cordierite has a low sintering property andcannot be densely sintered. Upon firing the cordierite by using thesintering assistant in combination, however, there can be obtained adense ceramics having a relative density of not smaller than 95%,preferably, not smaller than 96% and, more desirably, not smaller than97%. Besides, in the present invention, the element component isprecipitated on the grain boundaries of the cordierite crystal phase as,for example, a disilicate expressed by the following general formula(1a),(M¹)₂Si₂O₇  (1a)wherein M¹ is a rare earth element, Ga or In, or as an aluminosilicatesuch as celsian, anorthite or slawsonite expressed by the followinggeneral formula (1b),(M²) Si₂Al₂O₈  (1b)wherein M² is an alkaline earth element other than Mg.

Such a crystalline compound has a dense atomic arrangement. Uponprecipitating the crystalline compound on the grain boundaries, thegrain boundaries are reinforced, the Young's modulus is improved and thecoefficient of thermal expansion is decreased. Therefore, the ceramicsof the present invention does not exhibit a large coefficient of thermalexpansion owing to the use of the sintering assistant, but exhibits alarge relative density. Besides, since the disilicate or thealuminosilicate is precipitated on the grain boundaries, the ceramics ofthe invention exhibits a Young's modulus of not smaller than 130 GPa. Toprecipitate the disilicate or the aluminosilicate on the grainboundaries, the cooling after the firing must be conducted underpredetermined conditions as will be described later.

In the present invention., preferred examples of the rare earth elementinclude Y, Yb, Er, Sm, Dy and Ce. The rare earth element is contained inthe ceramics at a ratio of from 1 to 20% by weight and, particularly,from 2 to 15% by weight in terms of an oxide. Besides, the alkalineearth element other than Mg, or Ga or In is contained at a ratio of from0.5 to 10% by weight and, particularly, from 2 to 8% by weight in termsof an oxide thereof. When these element components are used in amountslarger than the above-mentioned ranges, the cordierite component reactsin an increased amount with these element components, causing thecoefficient of thermal expansion to increase. When the amounts of theseelement components are smaller than the above-mentioned ranges, on theother hand, the disilicate or the aluminosilicate does not precipitatein a sufficient amount on the grain boundaries of the cordierite crystalphase and, hence, the ceramics exhibits a decreased Young's modulus.Besides, the sintering property of the cordierite is not improved, and adense ceramics having a relative density of not smaller than 95% is notobtained.

The above-mentioned disilicate or the aluminosilicate is formed by thereaction of SiO₂ and Al₂O₃ only in the cordierite crystal phase with theelement components used as the sintering assistant. Therefore, thecordierite crystal phase in the ceramics does not necessarily have thecomposition expressed by the above-mentioned formula (I), but may have anonstoicheometrical composition which MgO or Al₂O₃ which is a residue ofthe reaction remains as a solid solution in the cordierite crystalphase.

An oxide of Sn or Ge can be effectively used as a sintering assistantmostly dissolving, however, in the cordierite crystal phase as a solidsolution. It is therefore desired that these oxides re used incombination with the above-mentioned components.

It is desired that the ceramics of the present invention contains atleast one silicon compound selected from the group consisting of siliconnitride, silicon carbide and silicon oxinitride, in addition to theabove-mentioned components. Here, the silicon oxinitride is a compoundhaving an Si—N—O bond, and is expressed by, for example, Si₂N₂O. Thesesilicon compounds are present as crystalline particles in the ceramics,and exhibit large Young's moduli by themselves. By containing thesecomponents, therefore, the Young's modulus can be further increasedwithout increasing the coefficient of thermal expansion of the ceramics.For instance, the ceramics containing such a silicon compound exhibits aYoung's modulus of not smaller than 150 MPa. In the present invention,the silicon nitride is most preferred among the above-mentioned threekinds of silicon compounds.

It is desired that the silicon compound for improving the Young'smodulus is contained in the ceramics in an amount of not larger than 30%by weight and, particularly, from 5 to 20% by weight. When this amountis larger than the above-mentioned range, the ceramics exhibits anincreased coefficient of thermal expansion deteriorating excellentproperties, i.e., low thermal expansion of the cordierite.

The ceramics of the present invention having the above-mentionedcomposition is a densely sintered product and has a relative density ofnot smaller than 95%, preferably, not smaller than 96% and, mostpreferably, not smaller than 97%, and having a coefficient of thermalexpansion at 10 to 40° C. of not larger than 1×10⁻⁶/° C. and,particularly, not larger than 0.5×10⁻⁶/° C. and a Young's modulus of notsmaller than 130 GPa and, preferably, not smaller than 140 GPa, and mostpreferably, not smaller than 150 GPa. Therefore, the ceramics isdeformed very little depending upon a change in the temperature, andexhibits a high rigidity. On account of these properties, the ceramicsof the invention is used as constituent parts in a variety of industrialmachines and particularly, in a vacuum apparatus, susceptor, vacuumchuck, electrostatic chuck and lithography apparatus used for theprocess for producing semiconductors. In particular, the ceramics of thepresent invention is very useful as parts constituting the lithographyapparatus for forming ultrafine circuit patterns on a semiconductorwafer.

The ceramics of the present invention may contain carbon in an amount offrom 0.1 to 2.0%by weight and, particularly, from 0.5 to 1.5% by weight.The ceramics containing carbon exhibits a black color and can beeffectively used for the applications where the light-shielding propertyis required, such as a mirror cylinder or a light-shielding plate in thelithography apparatus.

The ceramics of the present invention is very dense upon being preparedby firing under a predetermined condition or upon being prepared by theheat treatment under a predetermined condition after the firing, and hasa porosity of not larger than 0.1% and, particularly, not larger than0.08%, and a maximum void diameter of not larger than 5 μm and,particularly, not larger than 4.5 μm. The dense ceramics having such aporosity and a maximum void diameter, has a realtive density of, forexample, not smaller than 99.5% and, particularly, not smaller than99.9%, and excellent surface smoothness. According, the ceramics is mostsuited as parts which are coated on the surfaces thereof or as memberson which the surfaces are formed a thin film (0.1 to 10 μm) of TiN,Al₂O₃, diamond, diamond-like carbon (DLC) such as a vacuum chuck or amirror used for measuring the position of the stage (wafer-supportmember) in the lithography apparatus.

Preparation of the Ceramics

As a starting material for producing the low thermal expansion ceramicsof the present invention, there can be used a mixed powder of acordierite powder having an average particle diameter of not larger than10 μm, a sintering assistant and, as required, at least one siliconcompound selected from the group consisting of silicon nitride, siliconcarbide and silicon oxinitride or a carbon powder. In this case, insteadof using the cordierite powder, there can be used the powders of MgO,Al₂O₃ and SiO₂ being mixed together, so that the cordierite can beformed upon the fring.

The sintering assistant contains an element for forming theabove-mentioned disilicate or aluminosilicate, i.e., contains at leastone of alkaline earth element other than Mg, rare earth element, Ga andIn. The sintering agent is used as an oxide containing these elements,or as a carbide, a hydroxide or a carbonate that forms an oxide upon thefiring.

The sintering assistant and the silicon compound or carbon that isblended as required, are used so as to be present in the ceramics at theabove-mentioned ratios. In order to obtain the ceramics that thermallyexpand little exhibiting a coefficient of thermal expansion at 10 to 40°C. of, for example, not larger than 10×10⁻⁶/° C., the amount of thecordierite powder should not be smaller than 80% by weight of the wholeamount.

The above-mentioned mixture powder is homogeneously mixed together in aball mill or the like device, and is molded into a predetermined shape,the molding is effected by a known means, such as metal mold press, coldhydrostatic press, extrusion molding, doctor blade method or rollingmethod, In this case, it is desired that the molded article has adensity of not smaller than 55% from the standpoint of obtainingceramics having a high relative density.

Next, the molded article is fired and is then cooled to obtain the lowthermal expansion ceramics of the present invention.

The firing is executed in an oxidizing atmosphere or in an inertatmosphere such as of nitrogen or argon under normal pressure or underan elevated pressure of not lower than 100 kg/cm² or, particularly, notlower than 150 kg/cm². When the silicon compound such as siliconnitride, silicon carbide or silicon oxinitride us used, in particular,the firing should be effected in an inert atmosphere so that the siliconcompound is not oxidized.

The firing temperature is usually form 1100 to 1500° C. When the firingis conduced under normal pressure, however, it is desired that thefiring temperature is set to be relatively high, e.g., from 1300 to1500° C. and, particularly, from 1300 to 1400° C. When the firing isconduced under an elevated pressure, on the other hand, it is desiredthat the firing temperature is set to be relatively low, e.g., from 1100to 1400° C. and, particularly, from 1150 to 1400° C. this is becausewhen the firing temperature is low, a sufficiently densely sinteredproduct is not obtained and when the firing temperature is too high, onthe other hand, the starting powder in the molded article melts.

Due to the above-mentioned firing, the sintering assistant reacts withsome of the components in the cordierite to form a liquid phase.Accordingly, the sintering property of the cordierite is improved, and asintered product having a relative density of not smaller than 95% isobtained.

The above-mentioned black ceramics containing carbon can also beprepared by firing the starting powder in an atmosphere containingcarbon without mixing the predetermined amount of carbon powder into thestarting powder. For example, the molded article is arranged in a moldmade of carbon and is fired under an elevated pressure condition. Or,the molded article is buried in the carbon powder and is fired. By suchfiring, carbon infiltrates into the sintered product, thereby to obtaina desired black ceramics. In any case, it is desired that the firing forobtaining the black ceramics is conducted in an atmosphere of an oxygenpartial pressure of not larger than 0.2 atms. and, particularly, notlarger than 0.1 atms., while flowing a nitrogen gas, an argon gas or aCO/CO₂ gas. This is because, when the firing is conducted in anatmosphere having a high oxygen partial pressure, carbon reacts withoxygen and is released to the outside of the sintered product.

In the present invention, the firing is conducted under theabove-mentioned elevated pressure condition to obtain a very denselysintered product (relative density of not smaller than 9.5%) having aporosity of not larger than 0.1% and, particularly, not larger than0.08%, and a maximum void diameter of not larger than 5 μm and,particularly, not larger than 4.5 μm.

When the firing is conducted under normal pressure, too, there can beobtained a densely sintered product having a very small porosity and avery decreased maximum void diameter upon executing the heat treatmentunder an elevated pressure condition, the heat treatment is conducted ina gaseous atmosphere such as of nitrogen, argon or air under an elevatedpressure condition of not lower than 100 atms. at a temperature of from1100 to 1200° C. for about 1 to about 5 hours. The sintered productbecomes more dense due to the heat treatment conducted under such anelevated pressure condition. Accordingly, the relative density of thesintered product after fired under normal pressure needs not necessarybe larger than 95%, but needs be not smaller than at least 90%. That is,when the sintered product has a relative density of smaller than 90%, agas of a high pressure is trapped in the pores in the sintered product.Therefore, the voids cannot be decreased despite the heat treatment isconducted in a subsequent step under a high pressure condition.

After the above-mentioned firing or heat treatment is conducted under anelevated pressure condition, the sintered product is cooled down tonormal temperature. Here, in the present invention, it is important thatthe cooling down to at least 1000° C. is effected at a rate of notlarger than 10° C./min. and, particularly, at a rate of not larger than5° C./min. Owing to the gradual cooling, the disilicate or thealuminosilicate derived form the sintering assistant precipitates on thegrain boundary of the cordierite crystal phase, making it possible toobtain low thermal expansion ceramics having a high Young's modulus.When the cooling rate is larger than the above-mentioned range, thedisilicate or aluminosilicate is not precipitated in a sufficientamount, and the ceramics having a high Young's modulus is not obtained.

As described above, the low thermal expansion ceramics of the presentinvention has a small coefficient of thermal expansion and a highYoung's modulus, and can be effectively used as various parts in aprocess for producing semiconductors having high resolution circuits.Particularly, as parts in the exposure apparatus. FIG. 1 schematicallyillustrates a lithography apparatus used for a process for producingsemiconductors.

Referring to FIG. 1, a beam such as i-ray, excimer laser or X-ray,emitted from a source of light 1 travels through a mirror 3 in a lightguide passage 2, passes through an optical unit equipped with a reticulestage 4 on which the diagram of a circuit pattern is placed and anoptical element such as a lens 5, and falls on a silicon wafer 7 placedin a main body 6 of the lithography apparatus. The wafer 7 is placed onthe surface of an electrostatic chuck 8 which is placed on a stage 9.

In the lithography apparatus 6, the optical elements such as the sourceof light 1, reticule stage 4 and lens 5 are firmly supported by supportmembers 10, 11 and 12 secured to the lithography apparatus 6. The stage9 is moved at a high speed up to an exposure zone by drive systems suchas an X stage and an XY stage, so that the silicon wafer 7 held on theelectrostatic chuck 8 thereon is brought to a predetermined exposurezone.

The support members 10, 11 and 12 firmly supporting the above-mentionedoptical elements, and the members such as electrostatic chuck 8 andstage 9 holding the silicon wafer 7, shall not vibrate even slightlyduring the exposure to light or shall not be thermally deformed by achange in the temperature. This is because, vibration or deformation dueto heat deteriorates the precision of exposure, and makes it difficultto highly precisely form high resolution circuit patterns on the siliconwafer 7.

The ceramics of the present invention has a low coefficient of thermalexpansion, is deformed little by a change in the temperature and has avery high Young's modulus. Therefore, the ceramics of the invention hasa large resistance against vibration and is very useful as theabove-mentioned members.

EXAMPLES Experiment 1

A cordierite powder having an average particle diameter of 3 μm wasblended with powders of Y₂O₃, Yb₂O₃, Er₂O₃ or CeO₂ having an averageparticle diameter of 1 μm at ratios shown in Tables 1 and 2, followed bymixing in a ball mill for 24 hours. The mixed powders were then moldedin metal molds under a pressure of 1 ton/cm². The molded articles wereintroduced into a pot of silicon carbide, fired under the conditionsshown in Tables 1 and 2, and were cooled down to 1000° C. at averagecooling rates shown in Tables 1 and 2 to obtain various ceramics.

The thus obtained ceramics were polished and ground into a size of3×4×15 mm, and their coefficients of thermal expansion were measured at10 to 40° C. Relying upon the ultrasonic pulse method, furthermore,their Young's moduli were measured at room temperature. The results wereas shown in Tables 1 and 2.

The ceramics were also measured for their relative densities accordingto the Archimedes' method. The results were as shown in Tables 1 and 2.

TABLE 1 Composition (% by weight) Firing condition Coefficient Oxide ofTemper- Cooling Grain of thermal Young's Relative Sample rare earthature rate boundary expansion modulus density No. Cordierite element (°C.) (° C./min) crystal phase 10⁻⁶ (/° C.) (Gpa) (%) *1 95 Y₂O₃  5 1350 5no crystal 0.6 110 94 phase  2 92 Y₂O₃  8 1350 5 Y₂O₃.2SiO₂ 0.2 130 95 3 90 Y₂O₃ 10 1300 5 Y₂O₃.2SiO₂ 0.4 130 95  4 90 Y₂O₃ 10 1350 5Y₂O₃.2SiO₂ 0.3 130 96  5 90 Y₂O₃ 10 1400 5 Y₂O₃.2SiO₂ 0.4 140 96  6 90Y₂O₃ 10 1450 5 Y₂O₃.2SiO₂ 0.3 140 97  7 90 Y₂O₃ 10 1500 5 Y₂O₃.2SiO₂ 0.5140 97 *8 90 Y₂O₃ 10 1550 5 melt, — — — no crystal  9 90 Y₂O₃ 10 1350 2Y₂O₃.2SiO₂ 0.3 140 95 10 90 Y₂O₃ 10 1350 7 Y₂O₃.2SiO₂ 0.4 140 95 11 90Y₂O₃ 10 1350 10 Y₂O₃.2SiO₂ 0.5 130 96 *12  90 Y₂O₃ 10 1350 15 no crystal0.7 110 95 phase *13  90 Y₂O₃ 10 1350 20 no crystal 0.7 100 95 phase 1482 Y₂O₃ 18 1350 5 Y₂O₃.2SiO₂ 0.3 140 97 15 80 Y₂O₃ 20 1350 5 Y₂O₃.2SiO₂0.4 150 97 *16  75 Y₂O₃ 25 1350 5 Y₂O₃.2SiO₂ 1.3 150 97 Samples markedwith * lie outside the scope of the invention.

TABLE 2 Composition (% by weight) Firing condition Coefficient Oxide ofTemper- Cooling Grain of thermal Young's Relative Sample rare earthature rate boundary expansion modulus density No. Cordierite element (°C.) (° C./min) crystal phase 10⁻⁶ (/° C.) (Gpa) (%) 17 90 Yb₂O₃ 10 13505 Yb₂O₃.2SiO₂ 0.2 130 95 18 82 Yb₂O₃ 18 1350 5 Yb₂O₃.2SiO₂ 0.4 140 97 1990 Yb₂O₃ 10 1400 5 Yb₂O₃.2SiO₂ 0.3 140 96 20 90 Yb₂O₃ 10 1450 5Yb₂O₃.2SiO₂ 0.3 140 97 *21  90 Yb₂O₃ 10 1350 20 no crystal 0.7 120 95phase 22 91 Er₂O₃  9 1350 5 Er₂O₃.2SiO₂ 0.2 130 95 23 90 Er₂O₃ 10 1350 5Er₂O₃.2SiO₂ 0.2 130 95 24 90 Er₂O₃ 10 1400 5 Er₂O₃.2SiO₂ 0.2 130 95 2590 Er₂O₃ 10 1450 5 Er₂O₃.2SiO₂ 0.3 130 96 *26  90 Er₂O₃ 10 1350 15 nocrystal 0.7 120 95 phase 27 91 CeO₂  9 1350 5 Ce₂O₃.2SiO₂ 0.2 130 95 2890 CeO₂ 10 1350 5 Ce₂O₃.2SiO₂ 0.3 130 95 29 90 CeO₂ 10 1400 5Ce₂O₃.2SiO₂ 0.4 130 96 30 90 CeO₂ 10 1450 5 Ce₂O₃.2SiO₂ 0.4 130 97 *31 90 CeO₂ 10 1350 15 no crystal 0.7 120 95 phase Samples marked with * lieoutside the scope of the invention.

As shown in Tables 1 and 2, the oxide of a rare earth element was addedat a predetermined ratio to the cordierite, whereby a crystal phase ofdisilicate RE₂O₃.2SiO₂ (RE₂Si₂O₇, RE: rare earth element) wasprecipitated, the coefficient of thermal expansion was decreased to benot larger than 1×10⁻⁶/° C. and the Young's modulus could be increasedto be not smaller than 130 GPa. The Young's modulus increased with anincrease in the amount of addition thereof.

However, the sample No. 1 having a relative density of not larger than95% exhibited a Young's modulus that was smaller than 130 GPa. Thesample No. 16 containing Y₂O₃ in an amount of larger than 20% by weightexhibited a high Young's modulus but exhibited a coefficient of thermalexpansion that was larger than 1×10⁻⁶/° C.

In the sample No. 8 fired at a temperature of higher than 1500° C., themolded article melts, thereby, ceramics could not be obtained.

In the samples Nos. 12, 13, 21, 26 and 31 that were cooled down to 1000°C. at cooling rates greater than 10° C./min., the crystal phase ofdisilicate RE₂O₃.2SiO₂ did not precipitate. As a result, Young's moduliwere low and the coefficients of thermal expansion were great. It willthus be understood that precipitating the crystal phase of disilicateRE₂O₃.2SiO₂ on the grain boundaries is important for increasing theYoung's modulus and for decreasing the thermal expansion.

Experiment 2

Powders of various additives were mixed into the cordierite powder(having an average particle diameter of 2 μm and a BET specific surfacearea of 2 m²/g) so as to obtain compositions shown in Tables 3 to 6. Themixed powders were molded in metal molds under a pressure of 1 ton/cm².

Among the powders of additives, the silicon nitride powder, siliconcarbide powder and silicon oxinitride powder that were used possessed anaverage particle diameter of 0.6 μm, and the powders of other additivesthat were used possessed an average particle diameter of 1 μm.

The obtained molded articles were introduced into the pot of siliconcarbide, and were fired and cooled under the conditions of Tables 3 to 6to obtain sintered products. The samples were prepared from the sinteredproducts in the same manner as in Experiment 1, and were measured fortheir coefficients of thermal expansion and Young's moduli, and werefurther identified for their crystal phases other than the cordierite.The results were as shown in Tables 3 to 6. Relative densities of thesintered products were also shown in Tables 3 to 6.

TABLE 3 Firing Cooling Coefficient Composition (% by weight) temper-temper- Relative of thermal Young's Other Sample Cordi- Powdery additiveature ature density expansion modulus crystal No. erite note 1) (° C.)(° C.) (%) 10⁻⁶ (/° C.) (Gpa) phases *1 100 — — 1400 15 93 0.1 110 none*2 99.98 CaCO₃ 0.02 — 1400 15 94 0.2 115 none  3 99 CaCO₃ 1 — 1400 3 960.4 140 CaAl₂Si₂O₈  4 97 CaCO₃ 3 — 1400 3 98 0.2 140 CaAl₂Si₂O₈  5 95CaCO₃ 5 — 1400 3 98 0.2 145 CaAl₂Si₂O₈ *6 95 CaCO₃ 5 — 1400 15 98 0.3120 none  7 92 CaCO₃ 8 — 1400 3 99 0.3 160 CaAl₂Si₂O₈  8 90 CaCO₃ 10 —1400 3 99 0.4 160 CaAl₂Si₂O₈  9 88 CaCO₃ 12 — 1400 3 99 0.6 165CaAl₂Si₂O₈ *10  90 CaCO₃ 5 Y₂O₃  5 1400 15 98 0.3 125 none 11 90 CaCO₃ 5Y₂O₃  5 1400 3 99 0.3 155 CaAl₂Si₂O₈, Y₂Si₂O₇ 12 85 CaCO₃ 5 Y₂O₃ 10 14003 99 0.2 160 CaAl₂Si₂O₈, Y₂Si₂O₇ 13 75 CaCO₃ 5 Y₂O₃ 20 1400 3 99 0.4 165CaAl₂Si₂O₈, Y₂Si₂O₇ *14  75 CaCO₃ 5 Y₂O₃ 25 1400 3 99 1.2 160CaAl₂Si₂O₈, Y₂Si₂O₇ 15 93 CaCO₃ 5 Yb₂O₃  2 1400 3 98 0.3 160 CaAl₂Si₂O₈,Yb₂Si₂O₇ 16 90 CaCO₃ 5 Er₂O₃  5 1400 3 99 0.3 165 CaAl₂Si₂O₈, Er₂Si₂O₇Samples marked with * lie outside the scope of the invention. Note 1)Numerals represent amounts in terms of oxides.

TABLE 4 Firing Coefficient Composition (% by weight) temper- Relative ofthermal Young's Other Sample Cordi- ature density expansion moduluscrystal No. erite Powdery additive (° C.) (%) 10⁻⁶ (/° C.) (Gpa) phases*17  95 SrCO₃ 5 — — 1100 88 0.3 110 SrAl₂Si₂O₈ 18 95 SrCO₃ 5 — — 1250 950.2 140 SrAl₂Si₂O₈ 19 95 SrCO₃ 5 — — 1400 96 0.2 150 SrAl₂Si₂O₈ 20 95SrCO₃ 5 — — 1450 99 0.4 140 SrAl₂Si₂O₈ *21  95 SrCO₃ 5 — — 1550 melt 2293 SrCO₃ 5 Y₂O₃ 2 — 1400 99 0.3 130 SrAl₂Si₂O₈, Y₂Si₂O₇ 23 90 SrCO₃ 5Y₂O₃ 5 — 1400 100 0.3 160 SrAl₂Si₂O₈, Y₂Si₂O₇ 24 90 SrCO₃ 5 — Si₃N₄  51400 97 0.3 170 SrAl₂Si₂O₈, Si₃N₄ 25 85 SrCO₃ 5 — Si₃N₄ 10 1400 97 0.3170 SrAl₂Si₂O₈, Si₃N₄ 26 65 SrCO₃ 5 — Si₃N₄ 30 1400 95 0.4 180SrAl₂Si₂O₈, Si₃N₄ *27  55 SrCO₃ 5 — Si₃N₄ 40 1400 92 1.1 160 SrAl₂Si₂O₈,Si₃N₄ 28 80 SrCO₃ 5 Y₂O₃ 5 Si₃N₄ 10 1400 99 0.4 170 SrAl₂Si₂O₈, Si₃N₄,Y₂Si₂O₇ 29 75 SrCO₃ 5 Yb₂O₃ 5 Si₃N₄ 15 1400 99 0.4 170 SrAl₂Si₂O₈,Si₃N₄, Y₂Si₂O₇ 30 85 SrCO₃ 5 — SiC 10 1400 97 0.4 175 SrAl₂Si₂O₈, SiC 3185 SrCO₃ 5 — Si₃N₄O 10 1400 98 0.2 165 SrAl₂Si₂O₈, Si₂N₂O 32 95 BaCO₃ 5— — 1400 96 0.1 145 BaAl₂Si₂O₈, Y₂Si₂O₇ 33 94 BaCO₃ 5 Y₂O₃ 1 — 1400 960.4 140 BaAl₂Si₂O₈, Y₂Si₂O₇ 34 87 BaCO₃ 5 Y₂O₃ 8 — 1400 99 0.5 145BaAl₂Si₂O₈, Y₂Si₂O₇ 35 90 BaCO₃ 5 — Si₃N₄  5 1400 97 0.4 170 BaAl₂Si₂O₈,Si₃N₄ 36 85 BaCO₃ 5 — Si₃N₄ 10 1400 98 0.4 170 BaAl₂Si₂O₈, Si₃N₄ Samplesmarked with * lie outside the scope of the invention. The samples werecooled down to 1000° C. all at a cooling rate of 3° C./min.

TABLE 5 Firing Coefficient Composition (% by weight) temper- Relative ofthermal Young's Other Sample Cordi- ature density expansion × moduluscrystal No. erite Powdery additive (° C.) (%) 10⁻⁶ (/° C.) (Gpa) phases37 95 Ga₂O₃ 5 — — 1400 99 0.2 155 Ga₂Si₂O₇ 38 95 In₂O₃ 5 — — 1400 97 0.2160 In₂Si₂O₇ 39 90 Ga₂O₃ 5 Yb₂O₃ 5 — 1400 98 0.4 160 (Ga,Yb)₂Si₂O₇ 40 90Ga₂O₃ 5 — Si₃N₄ 5 1400 98 0.4 170 Ga₂Si₂O₇.Si₃N₄ Samples marked with *lie outside the scope of the invention. The samples were cooled down to1000° C. all at a cooling rate of 5° C./min.

TABLE 6 Firing Coefficient Composition (% by weight) temper- Relative ofthermal Young's Other Sample Cordi- ature density expansion moduluscrystal No. erite Powdery additive (° C.) (%) 10⁻⁶ (/° C.) (Gpa) phases41 92 SnO₂ 5 Y₂O₃ 3 — 1400 98 0.2 150 Y₂Si₂O₇ 42 96 GeO₂ 1 Y₂O₃ 3 — 140095 0.3 130 Y₂Si₂O₇ 43 92 GeO₂ 5 Y₂O₃ 3 — 1400 98 0.2 155 Y₂Si₂O₇ 44 89GeO₂ 8 Y₂O₃ 3 — 1400 99 0.4 140 Y₂Si₂O₇ 45 90 GeO₂ 5 Yb₂O₃ 5 — 1400 990.3 140 Yb₂Si₂O₇ 46 85 GeO₂ 5 Yb₂O₃ 5 Si₃N₄ 5 1400 99 0.4 170 Yb₂Si₂O₇,Si₃N₄ The samples were cooled down to 1000° C. all at a cooling rate of5° C./min.

As will be obvious from Tables 3 to 6, small Young's moduli wereexhibited by the samples Nos. 1, 2 and 41 containing no compound orsmall amounts of compound of an element for forming the disilicate orthe aluminosilicate. The sample No. 9 containing larger than 10% byweight of a compound of an alkaline earth element other than Mg,exhibited a coefficient of thermal expansion of higher than 0.5×10⁻⁶/°C. The sample No. 14 containing larger than 20% by weight of an oxide ofa rare earth element and the sample No. 27 containing larger than 30% byweight of the silicon nitride, exhibited coefficients of thermalexpansion that were not smaller than 1.0×10⁻⁶/° C. the sample No. 21fired at a temperature of higher than 1500° C. dissolved, and the sampleNo. 17 fired at a temperature of lower than 1200° C. exhibited arelative density of lower than 95% and a low Young's modulus.

In contrast with these Comparative Experiments, the samples of thepresent invention all exhibited coefficients of thermal expansion of nothigher than 1×10⁻⁶/° C. and Young's moduli that were not smaller than130 GPa. Among them, the samples to which silicon nitride, siliconcarbide and silicon oxinitride were added, exhibited Young's moduli thatwere not lower than 160 GPa.

The samples Nos. 1, 2 and 6 in which the disilicate crystal phase or thealuminum silicate crystal phase was not precipitated, all exhibitedYoung's moduli that were smaller than 130 GPa.

Experiment 3

A cordierite powder having a purity of not lower than 99% and an averageparticle diameter of 3 μm was blended with powders of oxides of rareearth elements Y₂O₃, Yb₂O₃, Er₂O₃ or CeO₂ having an average particlediameter of 1 μm at ratios shown in Tables 7 and 8, followed by mixingin a ball mill for 24 hours. The mixed powders were then molded in metalmolds under a pressure of 1 ton/cm² to obtain molded articles having arelative density of 58%.

The molded articles were introduced into the pot of silicon carbide oralumina, and fired in an open air at temperatures shown in Tables 7 and8 for 5 hours. The obtained sintered products were measured for theirrelative densities relying on the Archimedes' method. The results wereas shown in Tables 7 and 8.

After the firing, the heat treatment was further conducted in ahigh-pressure atmosphere under the conditions shown in Tables 7 and 8for one hour. The pressurized processing conditions were changed asshown in Tables 7 to 8 to obtain various ceramics.

Samples were prepared from the ceramics in the same manner as inExperiment 1, and were measured for their coefficients of thermalexpansion and Young's moduli, and were further identified for theircrystal phases other than the cordierite. Moreover, porosity and maximumvoid diameters were measured at room temperature. The results were asshown in Tables 9 and 10.

The maximum void diameter was measured by observing the texture at giventen points by using an electron microphotograph (magnification of 200times).

TABLE 7 Composition Relative (% by weight) Firing density Heat-treatingcondition Oxide of temper- after Temper- Cooling Sample rare earth aturefiring Atmos- ature Pressure rate No. Cordierite element (° C.) (%)phere (° C.) (atm) (° C./min) *1 90 Y₂O₃ 10 1375 97.5 Ar  500 2000 15  290 Y₂O₃ 10 1375 97.5 Ar  900 2000 5  3 90 Yb₂O₃ 10 1375 98.1 Ar  9002000 5  4 90 Er₂O₃ 10 1375 97.8 Ar  900 2000 5  5 90 CeO₂ 10 1350 95.5Ar  900 2000 5  6 90 Y₂O₃ 10 1375 97.5 Ar 1150 2000 5  7 90 Yb₂O₃ 101375 98.1 Ar 1150 2000 5  8 90 Er₂O₃ 10 1375 97.8 Ar 1150 2000 5  9 90CeO₂ 10 1350 95.5 Ar 1150 2000 5 10 90 Y₂O₃ 10 1375 97.5 Ar 1250 2000 511 90 Yb₂O₃ 10 1375 98.1 Ar 1250 2000 5 12 90 Er₂O₃ 10 1375 97.8 Ar 12502000 5 13 90 CeO₂ 10 1350 95.5 Ar 1250 2000 5 14 90 Y₂O₃ 10 1375 97.5 Ar1350 2000 5 15 90 Yb₂O₃ 10 1375 98.1 Ar 1350 2000 5 16 90 Er₂O₃ 10 137597.8 Ar 1350 2000 5 17 90 CeO₂ 10 1350 95.5 Ar 1350 2000 5 18 90 Y₂O₃ 101400 98.5 Ar 1400 2000 5 19 90 Yb₂O₃ 10 1400 99.1 Ar 1400 2000 5 20 90Er₂O₃ 10 1400 98.8 Ar 1400 2000 5 21 90 CeO₂ 10 1400 97.5 Ar 1400 2000 5*22  90 Y₂O₃ 10 1375 97.5 Ar 1450 2000 5 Samples marked with * lieoutside the scope of the invention.

TABLE 8 Composition Relative (% by weight) Firing density Oxide oftemper- after Heat-treating condition Sample rare earth ature firingTemperature Pressure No. Cordierite element (° C.) (%) Atmosphere (° C.)(atm) *23  90 Y₂O₃ 10 1375 97.5 Ar 1150 50 24 90 Y₂O₃ 10 1375 97.5 Ar1150 100 25 90 Y₂O₃ 10 1375 98.1 Ar 1150 500 26 90 Y₂O₃ 10 1375 97.8 Ar1150 1000 27 90 Y₂O₃ 10 1350 95.5 Ar 1150 1500 28 90 Y₂O₃ 10 1375 97.5Air 1150 2000 29 90 Y₂O₃ 10 1375 97.5 N₂ 1150 2000 *30  100  —  0 140097.2 Ar 1150 2000 31 99 Y₂O₃  1 1400 97.4 Ar 1150 2000 32 95 Y₂O₃  51375 97.8 Ar 1150 2000 33 86 Y₂O₃ 14 1375 97.7 Ar 1150 2000 34 82 Y₂O₃18 1375 97.5 Ar 1150 2000 35 80 Y₂O₃ 20 1375 97.6 Ar 1150 2000 *36  75Y₂O₃ 25 1375 97.5 Ar 1150 2000 *37  90 Y₂O₃ 10 1250 80.2 Ar 1150 2000*38  90 Y₂O₃ 10 1300 86.5 N₂ 1150 2000 Samples marked with * lie outsidethe scope of the invention. The samples were cooled down to 1000° C. allat a cooling rate of 5° C./min.

TABLE 9 Coefficient Grain Max. Void of thermal boundary Young's SamplePorosity diameter expansion × crystal modulus No. (%) (μm) 10⁻⁶/° C.phase (Gpa) *1 2.0 10.0 0.3 none 110  2 0.09 4.3 0.3 DS 130  3 0.03 4.10.5 DS 133  4 0.06 4.4 0.2 DS 130  5 0.1 3.9 0.5 DS 130  6 0.08 4.2 0.3DS 140  7 0.01 4.0 0.3 DS 140  8 0.05 4.3 0.2 DS 140  9 0.09 4.4 0.4 DS135 10 0.08 2.0 0.3 DS 140 11 0.01 1.8 0.3 DS 145 12 0.05 1.6 0.2 DS 14513 0.09 1.5 0.4 DS 135 14 0.08 0.8 0.3 DS 145 15 0.01 1.2 0.3 DS 145 160.05 0.9 0.2 DS 145 17 0.09 1.1 0.4 DS 140 18 0.07 0.7 0.4 DS 145 190.01 0.8 0.3 DS 150 20 0.05 1.1 0.4 DS 145 21 0.08 1.1 0.5 DS 140 *22 melted DS = RE₂O₃.2SiO₂ (RE: rare earth element)

TABLE 10 Coefficient Grain Max. Void of thermal boundary Young's SamplePorosity diameter expansion × crystal modulus No. (%) (μm) 10⁻⁶/° C.phase (Gpa) 23 1.2 18 0.3 DS 130 24 0.1 4.9 0.3 DS 140 25 0.09 4.8 0.3DS 140 26 0.08 4.6 0.2 DS 140 27 0.08 4.4 0.2 DS 140 28 0.07 4.3 0.3 DS140 29 0.07 4.3 0.4 DS 140 *30  0.08 3.8 0.2 DS 125 31 0.07 3.7 0.2 DS130 32 0.07 4.2 0.3 DS 135 33 0.07 4.1 0.5 DS 145 34 0.07 4.0 0.8 DS 14535 0.07 3.9 0.9 DS 150 *36  0.06 3.8 1.3 DS 155 37 4.5 30 0.4 DS 130 383.2 20 0.4 DS 130 DS = RE₂O₃.2SiO₂ (RE: rare earth element)

From Tables 7 to 10, it will be understood that upon treating thesintered product containing not less than 80% by weight of cordieriteand having a relative density of not lower than 90% under the conditionsof a pressure of not lower than 100 atms, and a temperature of 900 to1400° C., it is made possible to obtain ceramics having a furtherincreased relative density and a decreased porosity of not larger than0.1%.

However, the sample No. 22 that was treated at a temperature in excessof 1400° C. under an elevated pressure, was partly melted. The sampleNo. 1 that was treated at a temperature lower than 900° C. under anelevated pressure possessed a porosity of larger than 0.1%. The sampleNo. 36 containing larger than 20% by weight of an oxide of a rare earthelement exhibited a coefficient of thermal expansion in excess of1.0×10⁻⁶/° C. The sample No. 30 containing less than 1% by weight of theoxide of a rare earth element could be fired at a temperature range ofas very narrow as ±5° C.

The sample No. 23 heat-treated under a pressure of lower than 100 atms.possessed a porosity that was larger than 0.1%. When the samples Nos. 37and 38 having relative densities of smaller than 90% of before beingtreated under elevated pressure conditions were used, the porosity couldnot be decreased to be smaller than 0.1% and the maximum void diametercould not be decreased down to be smaller than 5 μm even after the heattreatment under the elevated pressure conditions.

Experiment 4

The cordierite powder having an average particle diameter of 3 μm wasblended with oxides of various rare earth elements having an averageparticle diameter of 1 μm, followed by mixing in a ball mill for 24hours (blended compositions are shown in Tables 11 and 12) in the samemanner as in Experiment 3. The mixed powders were press-molded, theobtained molded articles were buried in a carbon powder, subjected tothe hot-press firing in an argon stream having a predetermined oxygenpartial pressure, and were cooled down to 1000° C. at a cooling rate of5° C./min. to thereby obtain various sintered products. Tables 11 and 12show oxygen partial pressures, firing pressures and temperatures in thefiring atmosphere.

The obtained sintered products were measured for their relativedensities, coefficients of thermal expansion, Young's moduli, porositiesand maximum void diameters in the same manner as in Experiment 3. Theresults were as shown in Tables 13 and 14. Moreover, the carbon contentsin the sintered products were measured and the results were as shown inTables 13 and 14.

TABLE 11 Composition Firing O₂ partial Sample (% by weight) temperaturepressure Pressure No. Cordierite RE₂O₃ (° C.) (atm) (kg/cm²)  1 90 Y₂O₃10 1350 0.01 300  2 90 Yb₂O₃ 10 1350 0.01 300  3 90 Er₂O₃ 10 1350 0.01300  4 90 CeO₂ 10 1350 0.01 300  5 90 Y₂O₃ 10 1350 0.02 300  6 90 Y₂O₃10 1350 0.03 300  7 90 Y₂O₃ 10 1350 0.04 300  8 90 Y₂O₃ 10 1350 0.05 300 9 90 Yb₂O₃ 10 1350 0.05 300 10 90 Er₂O₃ 10 1350 0.05 300 11 90 CeO₂ 101350 0.05 300 12 90 Y₂O₃ 10 1350 0.10 300 13 90 Yb₂O₃ 10 1350 0.10 30014 90 Er₂O₃ 10 1350 0.10 300 15 90 CeO₂ 10 1350 0.10 300 16 90 Y₂O₃ 101350 0.20 300 17 90 Y₂O₃ 10 1350 0.30 300 18 90 Y₂O₃ 10 1400 0.05 300Samples marked with * lie outside the scope of the invention.

TABLE 12 Composition Firing O₂ partial Sample (% by weight) temperaturepressure Pressure No. Cordierite RE₂O₃ (° C.) (atm) (kg/cm²) *19  90Y₂O₃ 10 1350 0.05  50 20 90 Y₂O₃ 10 1350 0.05 100 21 90 Y₂O₃ 10 13500.05 300 22 90 Y₂O₃ 10 1350 0.05 500 *23  100 — 1350 0.05 300 24 99 Y₂O₃ 1 1350 0.05 300 25 95 Y₂O₃  5 1350 0.05 300 26 86 Y₂O₃ 14 1350 0.05 30027 80 Y₂O₃ 20 1350 0.05 300 *28  75 Y₂O₃ 25 1350 0.05 300 Samples markedwith * lie outside the scope of the invention.

TABLE 13 Coefficient Max. Void thermal Carbon Relative Young's SamplePorosity diameter expansion × Color content density modulus No. (%) (μm)10⁻⁶ (/° C.) exhibited (wt %) (%) (Gpa)  1 0.09 4.0 0.3 black 1.1 >99.9140  2 0.01 2.0 0.2 black 1.9 >99.9 145  3 0.02 2.7 0.3 black 2.0 >99.9140  4 0.03 2.9 0.4 black 1.9 >99.9 135  5 0.05 3.7 0.3 black 1.8 >99.9140  6 0.04 3.5 0.3 black 1.7 >99.9 140  7 0.02 2.5 0.3 black 1.5 >99.9140  8 0.05 4.0 0.3 black 1.0 >99.9 140  9 0.05 3.8 0.2 black 1.0 >99.9145 10 0.05 4.1 0.3 black 1.0 >99.9 140 11 0.05 4.2 0.4 black 1.1 >99.9135 12 0.08 4.0 0.3 black 1.0 >99.9 140 13 0.07 3.8 0.2 black 1.2 >99.9145 14 0.08 4.1 0.3 black 1.1 >99.9 140 15 0.09 4.2 0.4 black 1.0 >99.9135 16 0.09 4.0 0.3 black 0.2 >99.9 140 17 0.08 4.1 0.4 white 0.05 >99.9140 18 0.04 3.5 0.3 black 0.8 >99 140

TABLE 13 Coefficient Max. Void thermal Carbon Relative Young's SamplePorosity diameter expansion × Color content density modulus No. (%) (μm)10⁻⁶ (/° C.) exhibited (wt %) (%) (Gpa) *19  15.0 12.0 0.3 black 0.8 85 90 20 0.06 5.0 0.3 black 1.0 >99.9 135 21 0.02 3.0 0.3 black 1.2 >99.9140 22 0.01 2.0 0.3 black 1.4 >99.9 140 *23  0.11 3.8 0.2 black0.9 >99.0 100 24 0.07 3.7 0.2 black 1.1 >99.9 130 25 0.07 3.7 0.3 black1.2 >99.9 135 26 0.07 3.6 0.5 black 1.1 >99.9 145 27 0.05 3.5 0.9 black1.0 >99.9 150 *28  0.02 2.8 1.3 black 2.0 >99.9 155

It will be understood from the results of Tables 11 to 14 that uponeffecting the firing under an elevated pressure condition in a carbonatmosphere having an oxygen partial pressure of not larger than 0.2atms., there are obtained very dense black ceramics having smallporosities.

However, the sample No. 17 that was fired under a high oxygen partialpressure contained carbon in an amount of smaller than 0.1% by weight,and was not blackened. The sample No. 19 that was sintered under apressure of lower than 100 kg/cm² possessed a porosity higher than 0.5%and was not so dense. The sample No. 28 containing larger than 20% byweight of an oxide of a rare earth element exhibited a coefficient ofthermal expansion of larger than 1.0×10⁻⁶/° C. and the sample No. 23containing less than 1% by weight of the oxide of a rare earth elementexhibited a low Young's modulus and could be fired at a temperatureregion that was as very narrow as ±5° C.

It was confirmed that the crystal phase of disilicate represented byRE₂O₃.2SiO₂ (RE: rare earth element) had precipitated in the samplescontaining not less than 1% by weight of the oxide of the rare earthelement as measured by the X-ray diffraction.

Experiment 5

A square ceramic board having a side of 100 mm was prepared by usingmany ceramics obtained in Experiments 1 to 4, and was used as anXY-stage of a lithography apparatus, in order to examine the precisionof a marking position by exposure to X-rays. In this case, thetemperature of the atmosphere was set to be 25° C.±2° C.

When the ceramics having a coefficient of thermal expansion at 10 to 40°C. of not larger than 1×10⁻⁶/° C. and a Young' modulus of not smallerthan 130 GPa was used, the precision of exposure was very high, i.e.,100 nm or smaller. When the ceramics having a coefficient of thermalexpansion of larger than 1×10⁻⁶/° C. was used, on the other hand, theprecision of exposure was larger than 100 nm.

Furthermore, the ceramic board was vertically erected with its one endbeing secured. A pendulum having a weight of 100 grams was hung from aportion just over the other end (upper end) of the ceramic board, andwas naturally fallen down from an upper tilted direction to impart ashock to the upper end of the ceramic board from the transversedirection. Attenuation of vibration of the ceramic board at this momentwas measured by using a distorting gauge in order to measure the timeuntil the vibration has extinguished.

When the ceramic board having a Young's modulus of smaller than 130 GPawas used, a time of longer than 20 seconds was required until thevibration has extinguished. When the ceramic board having a Young'smodulus of not smaller than 130 GPa was used, this time was not longerthan 20 seconds. The time was shortened with an increase in the Young'smodulus. The time was not longer than 18 seconds when the Young'smodulus was not smaller than 150 GPa.

1. Low thermal expansion ceramics comprising: a cordierite crystalphase; and a crystalline compound phase precipitated in grain boundariesof the cordierite phase comprising (M¹)₂ Si₂O₇ or (M²)Si2Al₂O₃ (M²)Si ₂Al ₂ O ₈, wherein M1 M¹ is an element selected from the group consistingof rare earth elements, Ga and In, wherein M2 M² is an alkaline earthelement other than Mg, wherein when the element is a rare earth element,the element is contained in an amount of 1-20% by weight in terms of anoxide thereof, and when the element is Ga, In or an alkaline earthelement other than Mg, the element is contained in an amount of 0.5%-10%by weight in terms of an oxide thereof, and wherein the ceramics have arelative density of not less than 95%, a coefficient of thermalexpansion of not larger than 1×10⁻⁶/° C. at 10 to 40° C., and a Young'smodulus of not less than 130 GPa.
 2. Low thermal expansion ceramicsaccording to claim 1, further comprising not more than 30% by weight ofat least one silicon compound selected from the group consisting ofsilicon nitride, silicon carbide, and silicon oxinitride.
 3. Low thermalexpansion ceramics according to claim 1, wherein said ceramics has aporosity of not larger than 0.1% and a maximum void diameter of notlarger than 5 μm.
 4. Low thermal expansion ceramics according to claim1, wherein said ceramics contains carbon in an amount of from 0.1 to2.0% by weight and exhibits black color.
 5. A member made of the lowthermal expansion ceramics of claim 1 used for semiconductor processequipment.
 6. A member according to claim 5 used for supporting asemiconductor wafer in a lithography apparatus for forming highresolution circuit patterns on the semiconductor wafer.
 7. A memberaccording to claim 5 used for supporting an optical element in alithography apparatus for forming high resolution circuit patterns onthe semiconductor wafer.